Calibration of electronic instruments is necessary to ensure accuracy and consistency of measurements made by the instrument. Because characteristics, such as impedance values and amplifier gain, in electric measuring instruments change with time, temperature and other factors, the components of the instruments require periodic calibration to assure measurement accuracy. Prior to the advent of microprocessors, calibrators, i.e., standard sources used to calibrate electric measuring instruments, were generally from time to time calibrated by physically adjusting components in the calibrator so outputs of the calibrator comply with external standards. In complex calibrators, many internal physical adjustments, requiring time-consuming calibration routines often taking many hours, must be made.
With the advent of microprocessors and associated devices, such as random access memories and read-only memories, the calibration process and the apparatus required to achieve calibration have been greatly simplified. Random access memories store calibration correction factors and use software to compensate for gain and zero errors on multiple ranges of different measurements. Modern devices have used microprocessors and electronic memories to store constants based on comparisons to external standards for instrument calibration. The microprocessors and electronic computer memories store internal software and correction factors, almost to eliminate the need to remove covers of the devices. Hence, the need to physically adjust components within the devices has been virtually eliminated.
However, removing the need to make physical adjustments to components within a device such as a measuring instrument is only a small step to reducing the calibration procedure cost. For example, calibration of a multimeter having multiple functions and ranges or a precision source having multiple ranges still requires many different external stimuli to be applied to the instrument or source. The time required to apply these stimuli and the cost involved in supporting the extensive external standards which must be used in the calibration process are contrary to the modern trend of reduced instrument ownership costs.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for calibrating sources used to supply parameters, i.e., voltage, current and impedance, to electric measuring instruments.
Another object of the present invention is to provide a new and improved method of and apparatus for calibrating electric measuring instruments having multiple functions and ranges and/or sources having multiple ranges.
Still another object of the invention is to provide a new and improved apparatus for and method of calibrating electric measuring instruments and/or sources wherein the need to make physical adjustments to components within the calibrator is virtually obviated.
Still another object of the invention is to provide a new and improved electric measuring instrument calibrator having minimum need for external standards.
The problems involved in the use of the prior art manual and electronic techniques and apparatus for calibrating electronic instruments can be understood by considering the example of calibrating a precision DC voltage source. To calibrate such an instrument, whether it has internally stored software constants or requires manual adjustment, typically requires some type of external reference voltage, such as a standard cell, in combination with a null detector to make comparisons, and a multi-range ratio divider. This array of equipment is connected in various configurations to calibrate the millivolt to kilovolt ranges of the source.
The calibration procedure is laborious and repetitive, ideally suited for automation. A prior art DC precision instrument calibrating voltage source, as described in U.S. Pat. Nos. 4,541,065 and 4,585,987, was internally calibrated by a repetitive comparison process between internal responses and external standards for each range. By applying an external 10 volt standard to the source, internal measurements were made in the source and used to characterize the internal reference of the source. Similarly, a comparison to an external divider was made during the calibration process. The comparison to the external divider characterized the resistive ratio within the source. This simple application of artifact standards is needed to perform a full external calibration of the precision DC voltage source.
The greatest workload in calibrating modern electric measuring instruments involves calibrating DC and low frequency multimeter instruments in laboratories of companies that own such instruments or in laboratories where such instruments should be periodically shipped for calibration purposes. Because these multimeters are becoming more accurate as the workload demand increases, the cost of external calibration has or should correspondingly increase.
It is, accordingly, a further object of the present invention to provide a new and improved apparatus for and method of calibrating electronic multimeters.
Still another object of the present invention is to provide a new and improved relatively inexpensive apparatus for enabling electronic multimeters to be from time to time calibrated with a calibrating source that is from time to time automatically internally calibrated.
A further object of the present invention is to provide a new and improved apparatus for and method of calibrating a multimeter with a calibrator that is internally calibrated with a minimum number of external sources, to minimize the cost of ownership by minimizing calibration time of the calibrator.